The Penn Plax Cascade 300 GPH Hang On Filter Bio Sponge keeps the water in your aquarium clean and clear. This Cascade filter removes particles from your fish tank healthy and clean. This bio sponge also optimizes the colonization of beneficial aerobic bacteria which prevents cloudy water. Made specifically to fit the Cascade 300 Hang On Filter; this replacement bio sponge fits securely without gaps that lead to weaknesses in filtration strength. An exact fit keeps debris and contaminants from getting through your filtration system and back into water. Made of a durable poly-fiber; this bio sponge filter is made to last for several months under normal conditions. When used together; the Cascade 300 Hang On Filter Model CPF5 and Cascade 300 bio sponge filter cartridges provide maximum protection against water pollutants from harming your fish. Sold in a pack of 2.